1. Field of the Invention
The present invention relates to a display device, and more particularly, to a method for driving a display panel to form a desired waveform in driving a pulse width modulation (PWM) mode for adjusting a gray scale level.
2. Discussion of the Related Art
Generally, a gray scale level in a display device means an achromatic color system corresponding to colors from white to black.
To digitally drive such a gray scale, a PWM driving method is widely used. The PWM driving method is used for almost display devices regardless of passive driving or active driving.
FIG. 1 shows a circuit for adjusting a gray scale level of an organic electroluminescent (EL) panel according to the related art. As shown in FIG. 1, a PWM controller is added to a segment driving part of the organic EL panel.
The PWM controller added to the segment driving part is controlled in accordance with a segment signal, so that a pulse width of a signal applied to the display device is adjusted, thereby setting a gray scale level.
However, in such a PWM driving method, since a start time on all data lines of a data segment is fixed, a problem may occur in case of an organic EL panel.
Such a problem will be described with reference to the accompanying drawings.
FIG. 2 shows a PWM driving waveform of a related art display panel.
Referring to FIG. 2, the PWM driving method according to the related art is performed in such a manner that all signals are simultaneously turned on for one scan pulse width Ts and when a desired pulse width is obtained, a data line signal is shorted to turn off pixels.
Such a PWM driving method results in distortion of the waveform due to fluctuation of a voltage in an anode line in case of an organic EL panel.
In other words, on data line of a data segment of FIG. 1, a signal type of a first data line corresponds to a data line signal turned on within one scan pulse width, a signal type of a second data line corresponds to a data line signal turned on for a time period T2 only, and a signal type of a third data line corresponds to a data line signal turned on for a time period T1 only.
A signal waveform of an anode line applied to the panel through a driver shown in FIG. 1 is the same as signal waveforms of first, second and third anode lines.
In other words, since there is no signal waveform shorter than a signal waveform of the third anode line, the signal waveform of the third anode line occurs normally. However, a wave distortion A occurs in a signal waveform of the second anode line due to the signal waveform of the third anode line. Two wave distortions B and C occur in a signal waveform of the first anode line due to the signal waveforms of the second and third anode lines.
The wave distortions give an adverse effect to longevity of the display device due to luminance and momentary high voltage.
To solve such a problem, a PWM driving waveform as shown in FIG. 3 has been supposed.
Referring to FIG. 3, if no signal is input from the data line of the data segment to the display device, the display device floats the anode line to naturally consume charges inside the organic EL panel.
Momentary change in the signal waveform of the anode line is relieved as the charges inside the organic EL panel are naturally consumed. As shown in FIG. 3, it is noted that distortion of the signal waveform on the anode line is reduced.
However, the PWM driving method of FIG. 3 has a problem in that it is difficult to exactly adjust a gray level due to waveform of the PWM. Also, charges trapped inside pixels give an adverse effect to longevity of the display panel.